A foreign animal disease (fad) is an animal disease or pest, whether terrestrial or aquatic, not known to exist in the United States or its territories

A Foreign animal disease (FAD) is an animal disease or pest, whether terrestrial or aquatic, not known to exist in the United States or its territories.[1] When these diseases can significantly affect human health or animal production and when there is significant economic cost for disease control and eradication efforts, they are considered a threat to the United States.[2] Another term gaining preference to be used is Transboundary Animal Disease (TAD),[3] which is defined as those epidemic diseases which are highly contagious or transmissible and have the potential for very rapid spread, irrespective of national borders, causing serious socio-economic and possibly public health consequences.[4] An Emerging Animal Disease "may be defined as any terrestrial animal, aquatic animal, or zoonotic disease not yet known or characterized, or any known or characterized terrestrial animal or aquatic animal disease in the United States or its territories that changes or mutates in pathogenicity, communicability, or zoonotic potential to become a threat to terrestrial animals, aquatic animals, or humans."[5]

A foreign animal disease in the United States has the potential to threaten food security, cause production losses for livestock producers while significantly increasing livestock production costs through costly disease control measures, affect the income of livestock producers, disrupt movement of livestock and livestock products, cause animal welfare problems in affected animals, possibly cause public health issues, and cause environmental consequences with the wildlife populations.[3][4]